The biology of neoplasia is complex and associated with: (1)\intrinsic aberrations of the neoplastic cell behavior; (2)\changes in the surface membrane of the tumor cell with presentation of new or rearranged molecules; (3)\the initiation of recognitive events directed to the tumor cell surface; and (4)\the character of the response of the host. Our research addresses selected issues in the biology of neoplasia, specifically: (1)\the structure, cellular function, and molecular biology of candidate transformation-associated molecules. These currently include mammary tumor glycoprotein (MTGP), a new 126 kilodalton phosphotyrosinated glycoprotein, and poorly characterized cell surface proteins with immunological homology to known oncogene sequences or containing phosphotyrosine; (2)\the genomic basis for expression of these molecules and relationship of expression to the transformed phenotype; (3)\the tumor antigen MTGP and others as potential targets for in vivo imaging of experimental breast carcinomas with monoclonal antibody synthesized by cloned hybridomas; (4)\the recognition of these molecules and the host immune response to it; (5)\characterization and delineation of the genetic and functional restrictions in cellular cooperation required for immune induction of local activation of coagulation proteases and analysis of the bioregulatory properties of a discrete fibrinolytic peptide that is evolved, its effect on tumor cells, and the immune attack against tumors; and (6)\the molecular biology of receptor-mediated regulation of lymphoid function and the immune response by discrete serum lipoproteins and lipoprotein subsets to more clearly delineate an aspect of regulation of the host-immune response to neoplasia. (AG)